Charge coupled devices ("CCD's") are widely used in digital imaging devices such as optical scanners, fax machines, digital copiers, video cameras and digital cameras. CCD's are solid state devices formed using semiconductor fabrication processes well known in the art. See, for example, Solid State Imaging with Charge-Coupled Devices by Albert J. P. Theuwissen, Kluwer Academic Publishers, First Editionl 995, which is hereby incorporated by reference.
CCD's of the type used in scanning type imaging devices have a series of small photosensor elements or "pixels" positioned in a closely spaced linear array. (These photosensor elements are sometimes referred to as the "active" area of the CCD.) An imaging light beam from a scan line portion of the object is imaged onto this photosensor array. The imaging is typically done by a lens assembly having a substantial image reduction ratio, such that the image projected onto the linear photosensor array is much smaller, e.g., ten times smaller than the corresponding scan line portion of the object. Each pixel of the CCD generates an electronic data signal representative of the intensity of the light that is impinged thereon during a short period of time known as a sampling interval. In order for a CCD pixel to generate an accurate image it is necessary that it receive imaging light only from the corresponding portion of the object which is imaged. Any stray light (photons) which strikes the pixel during a sampling interval will result in a degradation of the image data generated by the pixel during that sampling interval. This degradation of the data, in turn, may cause blurring, ghosting or other image artifact in any image generated from the data. Blurring image degradation is referred to in the art as decreased MTF (modulation transfer function).
One way that stray photons enter a photosensor element is for light from a source other than the imaging light beam to strike a portion of the CCD adjacent to the photosensor element and reflect onto it. In color scanning devices having multiple closely positioned linear arrays the source of such stray photons is often an overlapping portion of the imaging light beam for an adjacent linear array. For example, in a color imaging device having a red, green, and blue linear sensor arrays provided on a single CCD, a portion of the red imaging beam may reflect off a region positioned close enough to the green sensor array to cause light from the red imaging beam to improperly enter the green linear sensor array.
One known technique for reducing the amount of stray light from adjacent imaging beams is to provide a microlens above each linear array using a clear plastic which is fabricated using conventional photolithography methods. The microlens helps to converge the imagining light beam onto the proper linear sensor array but does not entirely eliminate the problem of stray light from an adjacent imaging beam entering the wrong linear sensor array.
Light striking a CCD in regions other than the photosensor elements has also been known to cause problems. For example, light striking certain conductor regions formed adjacent to the photosensor regions has been known to cause errors in the data transmitted by the conductors. One way of eliminating this problem has been to cover the conductors with a light reflective photoshield which reflects light away from the conductors. Such photoshields may however have the effect of increasing the amount of stray light which is reflected onto the photosensor.
It would be generally desirable to provide a CCD assembly which reduces the amount of stray light which enters the respective photosensor elements thereof.